1. Field of the Invention
This invention relates to thymidylate synthase (TS) inhibitors. In particular, this invention relates to certain tosylproline analogs that are TS inhibitors, to methods of making them, to pharmaceutical compositions containing them, and to their uses.
2. Description of Related Art
Thymidylate synthase is an essential enzyme for virtually all living organisms. Together, TS, dihydrofolate reductase (DHFR), and serine hydroxymethylase form a biochemical functional unit, the thymidylate synthase cycle, that provides the sole de novo pathway for synthesis of the DNA base thymidine 5'-monophosphate (dTMP) from the RNA base uridine 5'-monophosphate (dUMP). Thus, TS and DHFR are both targets for anti-cancer drug development. Because TS is essential for all living species, and the TS gene is even found in many viruses, it is also a target for development of anti-parasitic, anti-bacterial, anti-fungal, and anti-viral agents.
TS is an especially attractive drug target for the following reasons: (i) it is the only enzyme in the cell in which 5,10-methylenetetrahydiofolate (mTHF) is oxidized during one-carbon transfer, and thus the associated binding site is unique; (ii) it is the rate-limiting step in the thymidylate synthase cycle; (iii) the TS gene is not as prone to amplification as is the DHFR gene, and therefore resistance by this mechanism should be less frequent; (iv) inhibition of either TS or DHFR causes `thymineless death` of the cell, but TS inhibitors do not cause purine depletion and the concomitant side-effects in normal human cells, as do DHFR inhibitors; and (v) tumor cells are especially sensitive to TS inhibition because the activity of TS is unusually high in rapidly proliferating cells, while this activity is barely detectable in non-proliferating cells: the levels of mRNA for TS increase up to 1000-fold higher in relation to DHFR in certain tumor cells, presumably to support the increased requirement for dTMP in rapidly dividing cells.
Since its discovery in 1957, 5-fluorouracil (5-FU), a prodrug that targets TS, has been used in the USA as a principal drug in chemotherapy aimed at cancers of internal organs. 5-FU enters the cell, becomes ribosylated and phosphorylated, and forms a covalent ternary complex with TS and mTHF. However, 5-FU has serious adverse side-effects because it affects at least seven other recognized biochemical pathways, and also becomes incorporated into DNA. See Takimoto C: Antifolates in clinical development. Semin Oncol (Supp 18):S18-40-S18-51, 1987, and references cited therein for the structures and properties of the compounds discussed in this section.
LY 231514 is a classical antifolate originally thought to be a pure TS inhibitor but later recognized to inhibit purine synthesis and DHFR activity. It must be polyglutamated by folylpolyglutamyl synthase for activity. LY 231514 has been studied in a number of clinical trials, with activity noted in non-small cell lung cancer, breast cancer, and colorectal cancer, at doses typically around 500-600 mg/m.sup.2 in a ten-minute IV infusion every three weeks.
Raltritrexed is a pure TS inhibitor that has been approved for use in Europe for metastatic colorectal cancer and is currently in Phase III testing in the USA. It must also be polyglutamated for activity. Raltritrexed has been reported to be active in colorectal cancer, breast cancer, ovarian cancer, and non-small cell lung cancer, with similar anti-tumor response rates and lower toxicity than 5-FU/-leucovorin. A typical dose is 3 mg/m.sup.2 in a fifteen-minute IV infusion every three weeks. See U.S. Pat. No. 4,992,550; Marsham P R, Wardleworth J M, Boyle F T, et al: Design and synthesis of potent non-polyglutamatable quinazoline antifolate thymidylate synthase inhibitors. J Med Chem: 42:3809-3820, 1999; and Cunningham D, Zalcberg J R, Rath U, et al: `Tomudex` (ZD1694): Results of a randomized trial in advanced colorectal cancer demonstrate efficacy and reduced mucositis and leucopenia. Eur J Cancer: 31A: 1945-1954, 1995.
ZD 9331, a derivative of raltritrexed, is a water-soluble pure TS inhibitor that is not polyglutamated within the cell. It is a more potent TS inhibitor than raltritrexed polyglutamates, and is presently undergoing Phase II trials. See U.S. Pat. No. 5,955,463; and Jackman A L, Kimbell R, Brown M, et al: The anti-tumor activity of ZD 9331, a non-polyglutamatable quinazoline thymidylate synthase inhibitor. In Purine and Pyrimidine Metabolism in Man VIII (ed. Sahota A and Taylor M) 185-188, Plenum Press, N.Y., 1995.
The disclosures of these and other documents referred to throughout this application are incorporated herein by reference.
Thus TS inhibitors are well-established as anti-tumor agents, especially for solid tumors.
Immunocompromised individuals, e.g. patients with HIV/AIDS, show opportunistic infections with fungi, with bacteria such as Pneumocystis carinii and Mycobacterium tuberculosis, and with other pathogens that can typically reduce life expectancy, often by many years. Other infections, e.g. parasitic infections, affect a significant proportion of the world's population. TS inhibitors with 100-fold selectivity for bacterial TS over human TS have been discovered, indicating that screening of compounds active against TS may lead to broad applications for TS inhibitors outside of cancer treatment, such as for anti-parasitic, anti-bacterial, anti-fungal, and anti-viral agents; and TS has been isolated and purified from a number of these organisms. TS from at least the following organisms is available: Pneumocystis carinii pneumonia is the most common lethal infection of AIDS patients. TS is the first protein that has been isolated and purified from Pneumocystis carinii. Tuberculosis has undergone a marked increase in the USA, among both HIV-negative and, especially, HIV-positive individuals. Multi-drug resistant forms of Mycobacterium tuberculosis are rapidly emerging. Toxoplasma gondii is an opportunistic pathogen frequently infecting immunocompromised individuals. Current therapies have serious side-effects. Plasmodium falciparum is the protozoan responsible for most cases of human malaria. There are strains resistant to most of the common anti-malarial drugs; and mefloquine, the most recent drug, has been recognized as a drug possibly inducing psychosis. Trypanosoma cruzi causes Chagas' disease. There is currently no satisfactory treatment. Leishmania major causes leishmaniasis, a disease affecting persons in the Mediterranean, Middle East, India, and China. Current therapy involves the use of toxic antimony compounds. Cryptococcus neoformans infection is frequently associated with AIDS. Cryptosporidium parvum causes chronic anti-microbial resistant gastrointestinal infections in immunocompromised individuals. In addition, as mentioned before, many viruses carry the TS gene in their DNA.
Thus TS inhibitors may prove valuable as anti-parasitics, anti-bacterials, anti-fungals, and anti-virals.